Owing to their well-balanced mechanical properties, excellent resistance against wear and excellent resistance against the heat, the acetal resins have extensively been used in such fields as automotive industries, electric machinery, building materials and the like.
However, the acetal resins have high degree of crystallinity and exhibit considerably higher mold shrinkage than general amorphous resins at the time when being molded by an injection-molding machine. Further, due to their anisotropic properties, the flowing acetal resins tend to develop deformation on the finally molded articles. Particularly, when the molded articles are flat boards, the deformation appears as warping.
To improve the abovementioned defects, various methods have been proposed, for example, a method to decrease the molecular weight of the acetal resins, or methods disclosed in Japanese Patent Publication No. 8816/62 and Japanese Patent Publication No. 8815/62 which are to decrease the melt viscosity by adding a polyalkylene glycol or an aliphatic alcohol to the acetal resins.
With the former method, however, the reduction in molecular weight causes the resins to lose the mechanical strength as well as other properties whereby the acetal resins lose their inherent merits. With the latter methods, on the other hand, the thermal stability of the acetal resins is markedly reduced deteriorating, at the same time, the mechanical strength.
The inventors of the present invention have found through their keen research that a composition which exhibits very small mold shrinkage and very small mold deformation (surface warping) at the time of molding operation, is obtained if a petroleum resin prepared by polymerizing a cracked petroleum fraction boiling between -15.degree. and 200.degree. C. and containing unsaturated hydrocarbons, and having a molecular weight in the range of 400 to 2500 and a second order transition temperature in the range of 35.degree. to 90.degree. C. is added to the acetal resin.
Astonishingly, the addition of the abovesaid petroleum resin to the acetal resin does not at all deteriorate various properties of the acetal resins such as thermal stability, moldability and thermal properties, but rather helps increase the mechanical properties.
The acetal resins themselves and the petroleum resins themselves are widely known compounds. However, it was not so far known that a resin composition obtained by blending these two resins exhibit the aforementioned excellent properties.